Disease Basics: The Disease Triangle

Lane Greer

Ever hear a plant sneeze or a tree cough? If a petunia had to say, “Ahhh,” where would its tongue be? Obviously, plant diseases are very different from animal illness. The most basic idea behind plant disease is that three things have to occur simultaneously. The graphic depiction of this is called the Disease Triangle, and it looks like this:

Powdery mildew infects bigleaf maple leaves in late summer.

Photo Credit: Lane Greer

These cut roses will have a shortened vase life because they’re infected with botrytis (also known as gray mold).

Photo Credit: Lane Greer

Calla lilies are highly susceptible to a root rot called Erwinia, a very stinky disease that kills the entire plant.

Photo Credit: Lane Greer

The host is the plant itself. Some plants can host any disease, while others only entertain specific ones. For instance, lilacs are highly susceptible to bacterial blight, a disease caused by a bacterium that also infects apple trees and pears, including Bradford pears – that ubiquitous tree found in every Wal-Mart parking lot (and everybody else’s parking lot, yard, street planting…you get the idea). Bradford pears are now so prevalent across the country (except in the Upper Midwest), that controlling bacterial blight on lilacs has gotten much tougher, since bacterial blight can find a host so easily. Another disease, cedar apple rust, can’t exist unless both apple trees and Eastern red cedars are present (although by “present” I mean within a mile or two of each other).

The pathogen is the disease. Diseases are most often caused by bacteria or fungi, which are found literally everywhere. But without the right host and the right environment, diseases are like Dracula in the sunlight – they can’t do any harm. (They can’t suck your blood either.) Pathogens can be very specific and infect only one or a handful of hosts, or they can be very broad and able to live on just about everything.

The environment includes the weather conditions needed for a pathogen to thrive. Bacterial blight of lilacs occurs in spring because the bacteria do well in wet weather and mild temperatures. Powdery mildew typically occurs later in summer, when temperatures are 70-80 degrees F during the day and nighttime humidity is high. Tomatoes and many other plants get late blight, which means that the disease occurs late in the season, so you’ll usually see symptoms in mid- to late summer.

Identifying a disease can be tricky business, but there are lots of resources available to help you determine what diseases are attacking your plants, including numerous books with helpful color pictures. Try the American Horticultural Society’s Pests and Diseases: The Complete Guide to Preventing, Identifying and Treating Plant Problems (DK Adult). You can also call your local Cooperative Extension Service office.

Once you know the pathogen, its hosts and the environment that favors the disease symptoms, you can use appropriate control methods. For starters, try plant resistant cultivars. If you see a tomato seed packet with “VFN” on it, for example, this means the tomatoes are resistant to verticillium wilt (V), fusarium wilt (F) and unfriendly (as opposed to beneficial) nematodes (N), all of which are huge problems for tomatoes. Plant resistance varies from “very resistant” (the plant rarely – or never – gets the disease), “somewhat resistant” (the plant usually doesn’t get the disease, but it might in a bad year), “somewhat susceptible” (the plant often gets the disease) and “very susceptible” (the plant almost always gets the disease and may be killed by it).

Another disease-control method is to just practice good sanitation, by getting rid of infected leaves or pruning plants to increase air circulation. We can also make sure our soil is fertile, since poorly nourished plants are more susceptible to disease, and give plants adequate moisture.

Lastly, we can use sustainable or conventional pesticides. (But as with any chemical, always be sure to carefully read and follow label instructions!) A word of warning: Chemical pesticides can aggravate some problems by getting you on the “pesticide treadmill,” where you treat one problem only to have another appear, which requires another treatment. Avoid chemicals by first using organic and sustainable approaches to handle disease problems. If you’ve done all you can and the disease isn’t going away, it may be time to take it up a chemical notch.

Tips

Often diseases (like powdery mildew) look worse than they really are. Many diseases are transitory and will be gone in a couple of weeks. Decide if you’re willing to accept a minimum level of disease. If not, use the suggested controls offered by the resources mentioned in this article.

To find out more about resistant cultivars, ask your garden center or local Cooperative Extension Service office for help, check online, or grab a big book like the Manual of Woody Landscape Plants by Dr. Michael A. Dirr (Ball Publishing).

Resources

Online resources, such as the plant pathology series offered by The Ohio State University, can help you identify a particular disease that may be attacking your plants.

Next Steps

The Dark Side of Black Spot
Roses and gardeners beware! Black spot can attack plants and leave your garden looking miserable. Here’s a look at how to protect your roses from this notorious leaf disease, as well as some black spot-resistant varieties.

Powdery Mildew in the Garden
Do your plants look like they’ve been covered in baby powder? Chances are they’ve got a case of powdery mildew. Learn how to keep this nasty fungus at bay and away from your plants.

Daylily Rust
Daylily rust is an exotic disease just introduced to the US in 2000. Learn how to identify, prevent and treat this relatively new daylily problem.